This type of device has long been known in principle. Striking-work timepieces having been common in times past because they are able to deliver auditory information concerning the current time during the night or in darkness. Since the arrival of radium, fluorescent dials, and other means for illuminating the dial making it possible to read the time adequately even in darkness, thus simplifying the construction, the creation, and the use of watches, the integration of striking-work mechanisms, due to the complication of the corresponding movements as well as the horological skill necessary for their production, has become a subject reserved for top-of-the-range timepieces. The manufacture of striking-work mechanisms, in particular gongs, until recently therefore was hardly developed and was based on a predominantly empirical know-how, which resulted in a variability of the sound of the produced striking-work watches.
In general, a striking-work mechanism of a watch is formed of at least one gong and a hammer, which strikes, following its actuation, against the gong so as to vibrate the gong. The gong often has the form of a circular arc so as to advantageously surround the movement of the watch by being placed in a plane parallel to the dial of the watch. It is normally formed by a wound metal wire, one of its ends being fixed to a blom stud, itself mounted rigidly on a plate of the watch, whereas the other end generally remains free. In some embodiments, the blom stud is connected to the middle so as to promote the sound transmission. The gong therefore acts as a resonator and the blom stud transmits the vibration of the gong to the plate of the watch, thus enabling a radiation of the sound waves, which are audible to the user in the form of a generated sound.
Normally, a striking-work watch comprises two gongs, the hour gong and the minute going, but may also comprise three or four gongs, which is known by the name of a chime, and may even comprise more gongs. The corresponding conventional gongs primarily differ in diameter and in length so that the sound generated is different, for example deep for the hour gong and high for the minute gong.
In order to produce the desired sound, in particular with regard to the tonal pitch thereof, its frequency composition and its difference compared to the sound produced by other gongs present in the watch, the amplitude, the period of the vibrations, and also the length of the gong have to be carefully adjusted. In fact, if the rigidity of the gong is excessive, the gong will not vibrate sufficiently. If, by contrast, the gong is too soft, the sound produced therefore will not be satisfactory.
These problems have only recently been studied in greater detail in the horological industry, and less empirical efforts in nature have been undertaken in order to develop gongs that produce a sound of which the frequency composition is determined beforehand.
In fact, the spectrum of a musical sound is generally composed of a base frequency, the first harmonic, and several harmonics which are integer multiples of the base frequency. The sound produced by an instrument may also comprise frequencies which are not integer multiples of the base frequency and are called partial frequencies. With regard to the horological field and the gongs of striking-work mechanisms, the vibration of the gong or the frequency composition of the sound thus produced normally comprises a number of partial frequencies. In particular, the base frequency perceived during the actuation of a striking-work does not correspond to the first characteristic frequencies of the gong itself. Hereinafter, the term base frequency means the tonal pitch perceived. This thus results from a combination of the spectral components contained in the vibratory behavior of the gong and the elements associated therewith in the chain of wave propagation at the origin of the sound.
The presence of partial frequencies in a sound can be perceived by a human as pleasant or unpleasant depending on the number of partial frequencies and their respective positions in the spectrum. The totality of the spectrum formed by the different harmonics and partial frequencies determines the tonal pitch of the sound as perceived by a human. In addition, the human perception of the sound depends not only on the number and position of the partial frequencies, but also on their amplitude. This may create discord or, on the contrary, harmony regarding the human perception of the sound produced. Generally, it is considered that the three first partial frequencies contribute to determining the tonal pitch and that the following partial frequencies determine the timbre of the sound, which is commonly called the richness, beauty or color of the sound.
In particular, the frequency composition of a sound produced by a gong, and therefore the frequency position of the harmonics and partial frequencies, can be influenced by the choice of the material, that is to say the physical properties, and also by the choice of the geometry of the gong. For a given geometry of the gong, the choice of the material allows to modify the position of the base frequency perceived and the color of the sound. The position of a harmonic modifies the vibration of the gong. The damping of one or more vibration modes can be influenced as a result. The frequency distribution of the partial frequencies always follows the same law in this case. The choice of the geometry of the gong for its part allows to modify the ratios between the partial frequencies, for example by modifying the rigidity of the gong.
Based on these facts, the above-mentioned efforts, which have been undertaken in recent years by the horological industry so as to enable the production of gongs generating a sound well determined beforehand, in particular in terms of its tonal pitch and frequency composition, concerned either the choice of material or the choice of geometry of the gong.
For example, document EP 2 107 437 proposes the use of precious materials such as gold or silver for the fabrication of gongs, this due to the physical properties of these materials with regard to the modulus of elasticity relative to their mass density, thus enabling the production of a sound having an increased number of partial frequencies. As mentioned above, the choice of the material does, however, not allow the frequency distribution of the partial frequencies relative to one another and thus remains a parameter having relatively little potential of alone achieving all the desirable characteristics of a gong.
Consequently, further efforts have been directed to the modification of the geometry of the gong, for example as described in document U.S. Pat. No. 7,746,732. Such a gong has a cross section that is variable at least in part along its longitudinal axis, for example continuously or by a succession of increases and decreases in its cross section. Although the quality of the sound can thus be improved, the fabrication of such a gong remains rather complicated and unsuitable for production on an industrial scale.
Another example of an effort targeting a specific gong geometry is disclosed in document CH 702 145. A gong as proposed in this document comprises a middle part having at least two different cross-sectional portions. This gong is based on the same principle as document U.S. Pat. No. 7,746,732, the variation of the cross section of the gong, apparently by simplifying the embodiment of the corresponding gong. However, it is not obvious whether this proposition allows to obtain an improvement in terms of the sound produced simultaneously with a simplification of the production process, also because the different cross-sectional portions are formed by metal wires having different diameters assembled by brazing or soldering. In fact, this should lead either to a rather complex production process in terms of the brazing or soldering method or to a reduction in the quality of the gong obtained.
The prior art solutions currently known therefore either have a rather complex structure preventing acceptable production of the corresponding gongs or do not allow to obtain a gong having all the desirable characteristics with regard to the quality to the sound produced.